Broadly defined, there are generally 5 main classes of flame retardants: metal oxides, hydroxides, and hydrates; halogens; (organo)phosphorus compounds; inorganic fillers; and intumescing materials. Regardless of the type of flame retardant, they generally follow one (or a combination) of three mechanisms for reducing fire hazards: vapor phase inhibition, solid phase char formation, and quenching/cooling.
For vapor phase inhibition materials, the flame retardant additive reacts with the burning material, such as a polymer, in the vapor phase. The vapor inhibition materials disrupt the production of free radicals at a molecular level and shut down the combustion process. This mechanism is commonly used with halogenated flame retardant systems. Char-forming flame retardant additives modify the decomposition pathway of the burning material by promoting the formation of a solid residue (char) on the material's surface and decreasing the amount of combustible volatiles. This char layer insulates the material, slowing pyrolysis, and creates a barrier that hinders the release of additional gases to fuel combustion. This method is commonly deployed by inorganic acids or acid precursors (e.g., phosphate salts) that induce crosslinking between, for example, polymer chains of the burning material. Nitrogen compounds, e.g. melamine, can be used in combination with phosphorous containing flame retardants to promote the formation of a porous char with enhanced thermal insulation and thermal stability through a synergistic mechanism observed when P—N bonds form prior to or during combustion.
Hydrated minerals are often used as halogen-free flame retardant systems commonly used for extruded applications like wire and cable. During combustion, the hydrated materials participate in an endothermic reaction to release water molecules that cool the burning material and dilute the combustion process.
The char forming mechanism can be enhanced by choosing compounds that intumesce. Intumescent compounds are insulating char forming materials that reduce heat and oxygen transport between the flame and unburned fuel source. In various embodiments, intumescent materials are comprised of (a) an acid source, which dehydrates, for example, a carbon source and/or the substrate, and is typically a phosphorus compound, such as ammonium polyphosphate, and (b) a carbonization agent or carbon source which chars during decomposition. Pentaerythritol and its derivatives have been most commonly used as a carbon source. The intumescent material may additionally comprise a blowing agent, which generates gas during decomposition. Blowing agents generally comprise a nitrogen compound. Melamine or urea have been used as blowing agents. In some embodiments, the blowing agent may be part of the acid and/or carbon source, for example, when the acid and/or carbon source contains N-containing groups, such as ammonium polyphosphate.